Support reading GitHub token from an environment variable

[hovsater]

I am proposing an enhancement to allow overriding the default token path by reading the GitHub token from an environment variable, in addition to the existing command line flag.

Would it be acceptable to add functionality to read the GitHub token from an environment variable? This would provide an alternative way to supply the token, potentially improving flexibility and security, as users often prefer environment variables for sensitive information.

One might argue that the same result can be achieved by passing the environment variable to the command line flag. While this is true, directly supporting environment variables may reduce the risk of exposing sensitive information in the command history. It also aligns with common practices in other tools and environments, where environment variables are a standard method for managing configuration and secrets.

I am of course willing to do the work to implement this feature if it is deemed useful.

Thank you for considering this request and for creating such a valuable tool. I appreciate all the work that has gone into it!